Most unprotected metal surfaces are subject to oxidative attack upon exposure to the atmosphere. The extent of oxidation and the form of deterioration (e.g. rusting, corrosion, discoloration, loss of lustre, staining and spotting, etc.) varies depending on the metal and the nature and severity of the conditions to which the metal is exposed.
Aluminum and high aluminum-content alloy products are particularly susceptible to oxidation in the form of water staining. Water stains, probably oxides and hydroxides of aluminum, appear as whitish blemishes on the surface of the metal and in the extreme cases surface pitting is also noted. The problem is not limited to exterior surfaces. Even with tightly rolled coils of aluminum sheet, moisture can condense at metal interfaces within the roll if the coil is exposed to atmospheric changes during shipment or outdoor storage, causing water staining throughout the coil and making it necessary to return the roll to the manufacture for reprocessing.
In addition to objectionable appearance, the presence of water stains cause numerous problems in subsequent fabricating operations such as rolling, stamping, drawing, extruding, cutting, welding, electroplating, lacquering and printing. Water stains on the surface of aluminum sheet cause frictional pickup on rolls and slipping and skidding of the sheet resulting in surface defects, considerable downtime, and operation of the mill at less that optimum speeds. Also, there may be considerable breakage in the production of thin films. In drawing operations, such as the production of beverage cans, pickup and galling of the die occur if the aluminum is water stained. Even if it is possible to form an acceptable can with water-stained aluminum application of color and print on the can may be less than satisfactory.
Several approaches have been used by the aluminum industry to eliminate, or at least minimize, the water staining problem. In some instances the metal is stored under conditions which deter the formation of water stains. This is not practical, however, with large quantitites of metal produced in a typical mill or where the metal must be shipped to a different location for final processing. The use of protective coatings is presently considered to be the most practical solution to the problem. Known protective coatings previously available to the industry have not been completely satisfactory, however, due to the numerous requirements which these products must satisfy.
In addition to providing a protective hydrophobic barrier on the surface of the aluminum, the coating must not interfere with subsequent fabrication and processing of the metal. It must also satisfy a number of other important criteria. First, it is essential that the protective composition not stain or otherwise attack the surface of the metal under conditions of application. Also, the protective composition must be capable of forming a continuous film on the metal surface which in some cases is uneven and irregular. The protective composition should be effective at low concentration and be in a form suitable for application to metal surfaces moving at high rates of speed. Since to facilitate application inert diluents are usually employed, the protective composition should have good compatibility with commonly used diluents and low volatility so that when the diluent evaporates a continuous film of the protective composition will remain on the metal surface to provide protection against stain. It is equally important, however, that the protective material not form such a permanent film that it cannot be removed by subsequent annealing. If the product is "burned-off" ash formation should be negligible.